retroactive no longer opens iphoto on my mac with Big Sur after previously working just fine

iPhoto will crash, if a slideshow is shown as selected, when you launch iPhoto. Even the Retroactive version cannot support slideshows in iPhoto.

If an incompatible item should be selected, you could try to rebuild the iPhoto Library:

• Make a backup copy of your iPhoto Library. You may need it.
• Hold down the key combination ⌥⌘ while double clicking the iPhoto Library you want to use.
• Keep holding down the keys firmly, until you are seeing the First Aid dialog.
• Select "Rebuild Library" and let iPhoto rebuild the library.
• If you are lucky, iPhoto will open again.

I would then let Photos for Mac migrate your library, so you can be sure to have access to your Photos, even when iPhoto will no longer work. As you can see, it is no longer safe to rely on patches and unsupported software. Do the migration while you can still open the library in iPhoto and compare it to the new Photos library. I am still using the patched version to be able to compare the original metadata and adjustments to what I am seeing in Photos, but my main workhorse is now Photos for Mac, supplemented by some third-party applications, like Luminar AI or Graphic Converter.

The best place to seek help with Retroactive is at the site of the makers of the app. It's almost never mentioned here. Further I also question the wisdom of trying to keep an app that's dead almost 6 years now going, especially when there are plenty of viable alternatives. It just seems to be delaying the inevitable.

Hello Yer_Man. Thanks for your comment. Apple's "iPhoto" may be dead to many people, but with it's wow factor, it's vastly superior to Apple's "photos". Multiple quality slideshows with classical music are quick and easy to produce - certainly better than "photos". Dare I say, people have come from interstate to see my slideshows. Please tell me what the viable alternatives are - I haven't seen one yet.

Further to my last comment. In "photos" I would have to go through all 300 pieces of classical music to choose one of appropriate length since it doesn't arrange music in time order as "iPhoto" does. In "photos" I have to export to make a slideshow. It would take me hundreds of hours (my annual leave) to produce exports for 200 slideshows. In "iPhoto" I simply right mouse button click run slideshow - no time at all (and then I can do other things on my annual leave). Besides these exports taking up hundreds of GB, they are only produced in 1080 lines of resolution - not 4K. This is why I like "iPhoto". Thanks again - Ross

Then I suggest you find an older Mac (or two!), put Mojave and iPhoto on it and nurse it until it dies.